The present invention relates to the packaging of consumer goods (food and non food products) in plastic containers.
In particular the present invention relates to improved package constructions comprising a base over which the product to be packaged is placed and a thermoplastic cover film which extends over the product and is welded to the base.
A substantial number of products including foodstuffs, such as cheese, meat, processed meat, poultry, fruit, vegetable, fish, pizza, etc., are currently sold in packages consisting of a base, such as a flat support or preferably a tray, on which the product to be packaged is placed, which is then overwrapped with a stretch film, such as, typically, stretch PVC (polyvinyl chloride) and stretch polyolefin films.
This packages are particularly useful when either a flat support is used as the base or when the base has a tray-like configuration and the product to be packaged is higher than the tray side-walls.
A stretch film is by definition a thermoplastic film that when applied under tension around a product elongates and conforms to the shape of the product to be packaged. Stretch overwrapping is generally carried out using either a horizontal stretch wrapper or an elevator-type stretch wrapper.
In the horizontal stretch wrapper the film is pre-stretched and applied over the product while kept under tension by a suitable grip system. The film is then folded longitudinally around the base supporting the product and sealed longitudinally below said base by means of a center-sealer. The film tubing is then transversally severed and the front and rear flaps thus obtained are folded and welded against the tubing surface by passing the package on a heated belt.
In the elevator-type stretch wrapper, the film is kept tensioned and stretched by raising the product placed on a suitable base against the film. Then the film is folded, both transversely and longitudinally, around the base supporting the product and bunch-sealed against the lower surface of said base by passing the package on a heated belt.
Depending on the type of film employed, passing on a heated belt may be insufficient to close the package by tack welding. In such a case, a pressure-assisted welding step is necessary, wherein a driven overhead pressure roller operates in conjunction with the heated belt.
In both cases, however, the welding that is obtained does not always provide for a hermetically sealed package. As a consequence thereof, purge or in general liquids that exude from the packaged product may leak from the package and contaminate the outside of the same package and/or of the other packages that are stored close to it. Furthermore the presence of a liquid in the tack welding area decreases the strength of the tack welding and the overwrapping film can easily unwrap or anyway the package becomes loose.
PVC is the film most commonly used in stretch overwrapping because it has, in addition to a remarkable elongation, also very good elastic properties, i.e. a good elastic recovery and a very low permanent deformation.
Alternatively, stretch polyolefin films are commonly employed such as those described for instance in EP-A-687,558, Japanese patent application publication no. 262673/1994 (Derwent Accession Number 94-337840), Japanese patent application publication no. 39973/1994 (Derwent Accession Number 94-103866), Japanese patent application publication no. 31882/1994 (Derwent Accession Number 94-086225), Japanese patent application publication no. 155210/1985 (Derwent Accession Number 85-239384), or Japanese patent application publication no. 327936/1992 (Derwent Accession Number 93-002817).
These stretch films may be manufactured by cast extrusion or co-extrusion, using either a flat or a circular film die that allows to shape the polymer melt into a thin film or tube; by heat or glue lamination of two or more cast films obtained as above; or by coating or extrusion coating of a cast film with one or more polymer layers. Alternatively, and preferably, these stretch films are manufactured by the blown film (or hot blown film) process wherein a mono- or multi-layer tube is formed and then, while it is still molten, is blown up like a bubble to generate a large diameter tube from a relatively small circular die.
Said stretch films, besides having remarkable elongation and preferably good elastic properties, may also be heat-shrinkable, i.e. they shrink when heated to a temperature that is above the, Vicat softening temperature of the film polymers but below their melting temperature. Said heat shrink feature is provided to the stretch films, by the process for their manufacture. Said process may involve extruding or co-extruding or extrusion-coating a so-called primary tube or sheet, that is quickly quenched, reheated to a suitable orientation temperature and oriented either mono-axially or bi-axially (trapped bubble process or tenter frame process). Or, as the so-called double bubble method described in EP-A-410,792, it may involve extruding or co-extruding the molten polymer to a hot blown film, heating the obtained film to a temperature above its orientation temperature and reinflating it by a blown bubble process. When the stretch film obtained by one of these methods is heated to a temperature that approximates the orientation temperature, it will shrink tending to return to its original dimension before orientation.
Examples of suitable heat-shrinkable stretch polyolefin films are for instance described in EP-A-286,430, EP-A-369,790, GB-A-2,154,178, EP-A-562,496, U.S. Pat. No. 5,460,861, etc.
Actually, in packaging, the stretch films, either PVC or the stretch polyolefin films, are used in the same way to overwrap the product placed on the flat support or in the tray, as indicated above.
When a heat-shrinkable stretch film is employed, a heat treatment following the bunch-sealing step improves the package appearance by tightly conforming the film to the packaged item. Besides the disadvantage of the poor hermeticity of the stretch overwrapped conventional packages, the cost of getting a package by stretch overwrapping, particularly when expensive polyolefin stretch films are employed, may sometimes be unacceptable at industrial level. A large surface of film is in fact needed to get
the overwrap of the base and the product placed thereupon, and
the overlapping between the edges to be welded together below the base itself.
As a consequence of the large surface of film required, an additional disadvantage of this packaging method resides in the large amount of plastic waste that is generated and that eventually needs to be disposed of.
Defensive Publication US-T-896 016 discloses a sealed package comprising a tray containing a foodstuff and a polymeric film attached to an outwardly extending flange of said tray by means of a heat sealed adhesive wherein the polymeric film has a tab portion extending beyond at least a portion of the perimeter of the flange to provide easy opening of the package by pulling on the tab.
Said polymeric film has a thickness of about 1 to 10 mils (25,4 to 254 xcexcm) and is not stretched over the foodstuff.
As known in the art, a package wherein a film is not stretched over the foodstuff results in a slack, unattractive package which becomes worse upon handling or upon storage under low temperature conditions.
Further drawbacks of said package are inherent in the manufacturing method requiring burdensome and expensive steps
of smearing said heat-sealed adhesive on the tray flange, and
of disposing of a substantial ring portion of film extending beyond the perimeter of the flange to provide material for cutting a tab therein.
FIG. 6 of U.S. Pat. No. 3,587,839 discloses a package consisting of a relatively rigid tray containing a packaged product wherein said tray is closed between an upper stretched elastic film and a lower heat shrunk film. This package involves a large waste of plastic material (i.e. the lower film) compared to a package where the upper film is attached to the tray rather than to an additional lower film.
In turn, the packaging method disclosed by U.S. Pat. No. 3,587,839 comprises the steps of
placing a heat shrinkable film on a platen,
placing a product or a relatively rigid tray containing the product to be packaged on said heat shrinkable film,
placing a stretchable elastic film on said product or on said relatively rigid tray containing the product to be packaged,
moving downward a film holding device to hold both films against the platen along a line around the periphery of said product, or of said relatively rigid tray containing the product to be packaged, spaced outwardly at a predetermined distance therefrom and from the sealing head so that the tensioning of said upper film occasioned by the subsequent lowering of said sealing head may be controlled and uniform,
lowering said sealing head while forcing said upper film into contact with said lower film along a line around and adjacent the periphery of said product or of said relatively rigid tray containing the product to be packaged,
energizing the electrical resistance of said sealing head to heat both films sufficiently to form a welded seal and to severe them along a continuous line around and adjacent the periphery of said product or of said relatively rigid tray containing the product to be packaged,
transporting the thus obtained package to a suitable station to effect an appreciable shrinkage of said lower heat shrinkable film.
In said method the sealing head has the double task of tensioning the upper film and of heating both films sufficiently to form a welded seal and to severe them. Thus, it is required sufficient time to allow said sealing head to become cold before performing the next tensioning a and sealing step otherwise the hot head pierces the upper film while tensioning it. The packaging speed is thereby substantially slowed down.
U.S. Pat. No. 2,147,384 claims a package comprising a relatively stiff opaque backing sheet of material capable of receiving printing impressions on the surface threreof, and a transparent facing membrane of thermoplastic elastic material stretched over and attached to the face of said backing sheet along the edge portions of said sheet and a membrane so as to provide a commodity receiving space between said sheet and membrane within the connected edges thereof, the edge portions of said backing sheet being corrugated and the edge portions of said transparent membrane being fused into the corrugations of said backing sheet. The backing sheet is said to be preferably composed of a cellulose material, preferably coated with casein. In contrast, no information at all are given about the composition of said transparent elastic rubber membrane and the elastic modulus thereof.
Even the packaging method and machine applied for manufacturing said package are not enabled by such document.
It is an object of the present invention to provide a hermetically sealed package for a product supported on a substantially rigid base as well as a method and a machine for manufacturing said package in a quick way.
It is another object of this invention to provide a hermetically sealed package for a product supported on a substantially rigid base as well as a method and a machine for manufacturing said package using less plastic material than necessary according to the prior art.
Preferably, the product to be packaged has a particularly high profile and is placed in a substantially rigid tray.
A first object of the present invention is to provide a hermetically sealed package comprising:
i) a substantially rigid base,
ii) a product to be packaged supported on said substantially rigid base,
iii) a stretch thermoplastic film extending over the product and welded, all around the product, to said substantially rigid base,
iv) said stretched thermoplastic film being heat welded to said substantially rigid base along a sealing line so as to form a hermetically sealed enclosure for the product.
In a preferred embodiment of the present invention, said substantially rigid base is a tray with a hollow or recessed center portion, upwardly extending side walls and a peripheral rim provided with a continuous flange and said stretched thermoplastic film extends over the product and is heat welded all around the tray along said sealing line on said continuous flange.
In an even more preferred embodiment of the present invention, the product to be packaged is higher than the tray side walls.
A second object of the present invention is to provide a method for manufacturing a hermetically sealed package for a product comprising:
a) providing a substantially rigid base,
b) placing said product to be packaged on said substantially rigid base,
c) stretching a thermoplastic stretch film all around said product,
d) welding said thermoplastic stretched film to said substantially rigid base all around the product,
e) cutting at least some of the stretched film extending beyond the outer perimeter of said substantially rigid base,
f) said stretching being performed by stretching and pressing said film against said substantially rigid base along a pressure line, and
g) said welding of said film to said substantially rigid base being performed by heating along a sealing line so as to form a hermetically sealed enclosure for the product.
In the package and in the method of this invention, preferably the stretch film is stretched in at least one direction by at least 10%, preferably by at least 15%, and even more preferably by at least 20%. Typically, the stretch film is stretched in both transverse and longitudinal directions. In such a case the stretch film may be stretched to a different degree in one direction compared to the other.
Preferably, when a heat-shrinkable stretch film is employed, said method further comprises heating said package thereby causing the heat-shrinkable stretch film to shrink.
In a preferred embodiment of the method according to the present invention the substantially rigid base is a tray with a hollow or recessed center portion, upwardly extending side walls and a peripheral rim provided with a continuous flange and said stretched thermoplastic film extends over the product and is heat welded all around the tray along said sealing line on said continuous flange.
In a more preferred embodiment of the method according to the present invention, the product which is placed on the upper surface of the tray center portion is higher than the tray side walls.
In an even more preferred embodiment, the product be packaged is fresh poultry and the method of packaging refers to poultry packaging.
A third object of the present invention is to provide a packaging machine for manufacturing a hermetically sealed package, said packaging machine comprising:
A) means for feeding a thermoplastic stretch film,
B) means for advancing said film over a substantially rigid base bearing a product to be packaged,;
C) means for stretching said film all around the product;
D) means for welding said stretched film to said substantially rigid base all around the product; and
E) means for cutting at least some of the stretched film extending beyond the outer perimeter of said substantially rigid base, wherein
F) said means for stretching comprises a first frame capable of stretching and pressing said film against said substantially rigid base along a pressure line, and
G) said means for welding comprises a second frame capable of heat welding said stretched film to said substantially rigid base along a sealing line so as to form a hermetically sealed enclosure for said product.
As it will be apparent, the present invention overcomes the drawbacks of the prior art in that the hermetically sealed package is manufactured by applying a stretch pressure on the film by means of the first stretching frame acting along the pressure line and by separately heat welding the film by means of a second welding frame acting along the sealing line. Thus, it is avoided any piercing of the film thereby improving the quality of the finished packages and the manufacturing speed is also improved.
Moreover, the waste of plastic material is reduced to a minimum.